Adhesively bonded cylindrical magnets comprising annular coils, and method of manufacture thereof

ABSTRACT

In a solenoid magnet assembly, and a method for manufacture thereof, the magnet assembly includes a number of concentrically aligned coils, each including a winding impregnated with a resin. Each coil is mechanically restrained so as to hold the coils in fixed relative positions relative to each other when forming the magnet assembly. The mechanical restraint can be formed by annular support sections bonded to the respective coils, lugs bonded to the respective coils, or by lugs that are at least partially embedded in a crust formed on a radially outer surface of the respective windings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the manufacture of cylindricalelectromagnets composed of a plurality of annular coils. The presentinvention finds particular relevance with respect to the manufacture ofsuperconducting coils for magnetic resonance imaging (MRI) systems, butmay be applied to other types of electromagnet, both superconducting andresistive.

2. Description of the Prior Art

Several methods for manufacturing cylindrical electromagnets have beenused in the past. Conventionally, a cylindrical former is produced, forexample of aluminum stainless steel or GRP, into which annular cavitiesof rectangular cross-section are formed. Coils of wire, for examplesuperconducting wire, are then wound into these annular cavities. Theresulting assembly may be impregnated with a thermosetting resin toretain the wire within the coils in position.

More recently, methods have been proposed in which coils are wound intocavities within a mold, with a supporting structure placed on theradially outer surface of the coils, and the resulting structureimpregnated with thermosetting resin. The mold is then removed leavingthe impregnated coils bonded to the supporting structure.

Co-pending United Kingdom patent application No. GB0912367.0 describes asolenoidal magnet arrangement in which the coils are bonded by theirradially outer surfaces to a radially outer mechanical supportstructure.

United States Patent Application Publication No. 2007/0247263 describesanother solenoidal magnet arrangement in which the coils are bonded bytheir radially outer surfaces to a radially outer mechanical supportstructure, and similar arrangements in which the supporting structure isplaced in the mold first, with the coils being wound over the supportingstructure.

A drawback of these methods is that the relative positions of the coilscannot be adjusted. While it may be theoretically possible to remedydeficiencies in the resultant magnetic field by adjustment of therelative positioning of certain coils, in practice this is not possiblewith coils structures which have been bonded to a support structure byresin impregnation.

Similarly, it has been found impractical to remove a single coil fromsuch an assembly for repair or replacement, leading to scrapping ofcomplete cylindrical magnet coil assemblies, even where only a singlecoil is defective.

Any new method of assembling annular coils must ensure that the coilsare accurately positioned relative to one another, and that the coilswill not move under the significant forces which they are subjected toin use.

Support structures for coils of superconducting solenoid magnets aredescribed in U.S. Pat. Nos. 4,896,128 and 4,467,303. In each case, acoil is clamped onto a partial outer former by compressing the outerradial extremity of the coil between a step formation formed on theradially inner surface of the partial former and a clamp piece or clampring. In use, axial forces of several tonnes will act on the coils. Asthe coils are restrained only at their radially outer extremity, bycompression at their axially outer extremities, severe stresses willbuild up within the coils. At least in U.S. Pat. No. 4,467,303, thecoils are restrained by thermal shrinking of the outer support structure(e.g. column 4 lines 7-18). The resultant hoop, radial and axialstresses may cause damage to the structure of the coils, by cracking theimpregnating material. Resulting movement of turns of the coils mayresult in a quench. The coils will be subjected to severe mechanicalloading at the points of contact of the coil with the step formation andthe clamp pieces or clamp ring. The present invention providessuperconducting solenoidal magnet arrangements in which mechanicalloading of the coils is not concentrated at localized points of contact.Rather, according to the present invention, each coil is bonded over itsradially outer surface to an outer mechanical support structure. Thisavoids any local points of high mechanical stress, for example highshear stress within the coil structure such as caused by conventionalcoil supporting formations.

The mechanical load resulting from the axial forces on each coil isborne by a large surface area of the coil structure, where it is bondedto the radially outer mechanical support structure.

The term “solenoidal” is generally applied to describe cylindricalmagnets made up of individual coils, although such cylindrical magnetsmay not be “solenoids” in the pure sense.

SUMMARY OF THE INVENTION

The present invention provides methods for manufacturing cylindricalmagnets, and such magnets, that have annular coils bonded on theirradially outer surfaces to a sectional radially outer mechanical supportstructure. In preferred embodiments, each annular coil is bonded to itsown annular section of the radially outer mechanical support structure.

In an example method of the present invention, individual coils areadhesively bonded to corresponding sections of the support structure toform composite sections, each of these composite sections including aresin impregnated coil bonded to an annular section of the supportstructure.

The annular sections of the support structure may be of a material suchas aluminum or a composite material such as glass fiber impregnated withthermosetting resin.

The individual annular sections of the support structure are then fixedtogether by suitable mechanical fasteners, such as tie-rods, spacers andfasteners, to form a complete cylindrical magnet coil structure.

Alternative embodiments of the method of the invention may involveattaching the coils to the annular sections using the impregnatingresin, which may be performed during the resin impregnation step: in asingle impregnation step; or in two impregnation steps, as will bediscussed below.

In a certain embodiment, employing only a single impregnation step,coils or wire are wound into a mold tool journal. The winding may beperformed separately for each coil, or multiple coils may be wound atonce in a batch process.

The coils are then inserted into corresponding annular sections of themechanical support structure. Arrangements, well known in themselves tothose skilled in the art, are provided to ensure that each coil isconcentric with the corresponding annular section of the supportstructure. At least the radially inner surface of the annular section isprepared for bonding to resin. This may simply be by ensuring that thesurface is clean, but preferably involves some texturing of the surface.If the annular section is of aluminum, the preparation may be ananodizing step.

The resultant structure may be regarded as a mold, comprising the moldjournal which carries the coil, the annular section of the supportstructure with the coil placed radially between the journal and theannular section, with appropriate tooling to ensure that the annularsection and the coil are retained concentrically, and walls of the moldto complete the impregnation cavity.

The mold structure is inserted into a trough. A thermosetting resin isadded in the conventional manner, to impregnate the coil and bond thecoil to the annular section. As is conventional in itself, a fillermaterial such as glass fiber cloth or glass beads may be placed in a gapbetween a radially outer surface of the coil and a radially innersurface of the annular section, if required. During the impregnationstep, any such filler material will be impregnated and will form part ofthe resultant structure, with the coil bonded to the annular section.

The thermosetting resin is then allowed or caused to cure. The troughand journal are removed, leaving the coil and annular section adhesivelybonded together by the thermosetting resin, with any filler materialprovided being embedded in a layer of thermosetting resin radiallypositioned between the coil and the annular section.

In another embodiment, which involves two impregnation steps, the coilsinitially are wound into a mold tool. The winding may be performedseparately for each coil, or multiple coils may be wound at once in abatch process. Preferably, a filler layer such as glass fiber cloth orglass beads is placed over the radially outer surface of the coil withinthe mold.

Each coil is then impregnated within the corresponding mold byintroduction of a thermosetting resin in the conventional manner. Thethermosetting resin is then allowed or caused to cure, and the resultingimpregnated coils are removed from the mold. The filler layer, if any,will have formed a resin-impregnated “crust” on the radially outersurface of the coil.

The outer surface of the crust of each coil is preferably then machinedto a specified diameter.

The coils are then inserted into respective annular sections of thesupport structure. Arrangements, well known in themselves to thoseskilled in the art, are provided to ensure that each coil is concentricwith the corresponding annular section of the support structure.Typically, a narrow annular gap is provided between the radially outersurface of the crust and the radially inner surface of the annularsection. At least the radially inner surface of the annular section isprepared for bonding to resin. This may simply be by ensuring that thesurface is clean, but preferably involves some texturing of the surface.If the annular section is of aluminum, the preparation may include ananodizing step.

Thermosetting resin is then introduced into the gap between coil crustand the annular section of the support structure. The thermosettingresin is then allowed or caused to cure. The journal is removed, leavingthe coil and annular section adhesively bonded together by thethermosetting resin, with the crust layer radially positioned betweenthe coil and the annular section.

Once a complete set of coil and annular section composites have beenmanufactured, they are mechanically combined together to form a completecylindrical magnet coil structure, for example by using tie bars andspacers. The relative positions of the individual composites, and thecoils they contain, are determined by either adjusting or setting thespacing of the tie bars, or the size of spacers used. For example,differently-sized spacers may be available, and/or shim pieces may beadded to the spacers to adjust the alignment and spacing of the coils.

The present invention effectively provides axially divided coil formerstructures, which are mechanically assembled into a modular magnet coilstructure. Such an arrangement enables the magnet coil structure to bedisassembled, permitting the replacement or rework of a single coilwithin a magnet structure. This is not possible with arrangements whichhave all coils bonded onto a single former.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show radial and axial cross-sections through a cylindricalmagnet assembly according to an embodiment of the invention.

FIG. 1C shows an enlargement of a part of FIG. 1B.

FIG. 1D shows a cut-away view of a composite section depicted in FIGS.1A-1C.

FIG. 1E shows a spacer as also shown in FIG. 1A.

FIG. 2A shows an axial cross-section through a cylindrical magnetassembly according to another embodiment of the invention.

FIG. 2B shows a cut-away view of a composite section depicted in FIG.2A.

FIG. 2C shows a cut-away view of a spacer as also shown in FIG. 2A.

FIG. 3A shows an axial view of a coil with attached lugs for acylindrical magnet assembly according to another embodiment of theinvention.

FIG. 3B is a cut-away view of a coil with a crust and lugs.

FIGS. 3C and 3D show views of lugs as may be used in coils such as shownin FIG. 3A.

FIG. 3E shows an example arrangement for adhesively bonding lugs toimpregnated coils.

FIG. 3F shows a partial view of an assembly of coils such as illustratedin FIG. 3A.

FIG. 4 shows a cut-away view of a coil according to another embodimentof the present invention.

FIG. 5 shows a part-axial cross-section through a tooling arrangementuseful for performing resin impregnation in a method of the presentinvention.

FIG. 6 illustrates an axial cross-section of a magnet according to anembodiment of the invention.

FIG. 7 shows an axial cross-section through another tooling arrangementuseful for performing resin impregnation in a method of the presentinvention.

FIG. 8 illustrates an annular support section made up of two parts, asemployed in certain embodiments of the invention.

FIG. 9 shows a partial radial cross-section through a toolingarrangement useful for performing resin impregnation in a method of thepresent invention.

FIG. 10 shows an example of a mechanical coil alignment adjuster.

FIG. 11 illustrates a coil and square support section.

FIG. 12 illustrates an arrangement, similar to that shown in FIG. 3F,which may be used to assemble supported coils such as that illustratedin FIG. 11 into a magnet assembly.

FIG. 13 shows an adjustable coil retention arrangement which may be usedin assembling supported coils such as that illustrated in FIG. 11 into amagnet assembly.

FIG. 14 shows an arrangement useful for adjusting coil concentricity incertain embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Certain specific embodiments will now be described with reference to theaccompanying drawings.

Molded Coils with Tie Bars

FIGS. 1A-1B respectively show radial and axial cross-sections through acylindrical magnet assembly according to an embodiment of the presentinvention. FIG. 1C shows an enlargement of the part of FIG. 1Bidentified as IC. FIG. 1D shows a cut-away view of a composite sectioncomprising a coil and an annular support section.

As discussed above, impregnated coils 10 are formed and adhered toannular sections 12 of a support structure, by a method such asdescribed above. In this embodiment, each annular section comprises aradially directed flange 14 extending radially away from a cylindricalportion 16 which is bonded to the coil 10. Coil 10 itself may include awinding 54 and a crust layer formed of a resin-impregnated fillermaterial 88.

Each radially directed flange 14 is provided with a number ofthrough-holes 18. A number of tie-rods 20 are provided, extendingthrough through-holes 18 at positions distributed around the radiallydirected flange 14. Spacers 22 are provided over the tie-rods, betweenadjacent flanges 14, to define and maintain a desired spacing betweenthe coils 10. As shown in FIGS. 1A and 1E, these may be arcuateportions, having one or more elongate arcuate holes 25 for the tie-rods20 to pass through. Alternatively, a single spacer 22 may be formed as acomplete ring. An alternative of placing tubular spacers individuallyover the tie rods 20 is possible, but is not presently preferred as theincreased surface area of arcuate spacers such as illustrated improvesthe rigidity of the structure as a whole. The spacers should be made ofa non-magnetic material such as aluminum or resin-impregnated fiberglasscloth.

In an alternative embodiment, these spacers may be made of a magneticmaterial such as iron, provided that the shape, position andmagnetization of the spacers is taken into account when designing themagnet, for example to create the required magnet field in an imagingregion, or to act as shielding of the stray field for a magnet designthat has no active shielding coils.

Retainers 24 are provided at each end of the tie-rod. For example, a nutand washer may be provided onto threaded ends of the tie-rod. Thetie-rods 20 and the retainers 24 must be of sufficient quality and of anappropriate material, and provided in sufficient number, to restrain allforces which are experienced by the coils in use, and any forces whichthe coils may experience in transit. The tie rods and retaining meansare preferably of a non-magnetic material. Stainless steel and aluminumcomponents have been found to have acceptable properties for use in thisapplication taking into consideration the thermal and mechanicalproperties required for the specific parts.

Assembly of the structure of FIGS. 1A-1C may proceed as follows. Eachcoil 10 is separately bonded onto a corresponding annular section 12 toform composite elements, one of which is partially illustrated in FIG.1D. The composite elements are stacked over a jig which ensures correctalignment of each coil onto the axis A-A of the magnet assembly, asshown in FIG. 1B. The jig may comprise a suitable number of demountablemandrels mounted on an axis. Spacers 22 are accurately dimensioned, forexample being ground flat to a close tolerance on each of their axialends 22 a. This ensures alignment and correct relative positioning ofthe coils when the structure is assembled.

Advantages provided by such an embodiment include the following. Directmanual or mechanical handling of the coils is not required—each coil ishandled by the corresponding radially directed flange 14 of the annularsection 12. The use of an alignment jig when stacking the coils andassembling the structure of FIGS. 1A-1C ensures that the coils areaccurately aligned, and ensures consistent and repeatable alignment ofcoils within magnet assemblies. If one coil is damaged, during assemblyor later on, it is a relatively simple task to release the fasteningmeans 24, remove coils 10 and spacers 22 for access to the damaged coil,and to replace the damaged coil. The remaining coils and spacers maythen be replaced, to re-construct the magnet assembly. The alignment jigshould be used for alignment of coils during re-construction of themagnet coil assembly.

In an example 3 Tesla magnet of the type illustrated in FIGS. 1A-1E, thecentral coil will experience no net axial force. The outer coils, on theother hand, may each experience an axial force of 400 tons directedtoward the axial center of the magnet. The intermediate coils mayexperience an axial force of 100 tons directed toward the axial centerof the magnet. The mechanical support structure, and the structure ofthe coils themselves, must be capable of withstanding such forces over along period of time.

Arrangements must be provided for electrical connections to be providedto and between the coils, but are not illustrated, for clarity. Ends ofeach coil winding 54 are kept accessible during the impregnationprocedure by conventional methods.

If additional strengthening of the assembly is required, for example tocounteract any bending of the tie bars due to the electromagnetic forcesand the differential thermal contraction of the components, one optionmay be to attach additional rings. Such rings may provide improvedmechanical strength to the ends of the structure, if required, and mayalso be positioned where required to counteract the effects of theelectromagnetic forces and differential thermal contraction on thesupport structure. Such end-rings are preferably of non-magneticmaterial attached to axial ends of the magnet coil assembly by tie-rods20 and retainers 24 as for an annular section 12.

Mold Coil Spacer Rings

For magnets with relatively high axial electromagnetic forces analternative embodiment using spacer rings (FIG. 2A) in the place of tiebars (FIG. 1B) may be considered better suited for supporting the magnetcoils. Advantages of using spacer rings include (1) the outer diameterof the support structure is reduced because the spacers are closer tothe coils and (2) any bending moment acting on the support structure maybe reduced.

FIG. 2A shows an axial cross-section of an example magnet coil assemblyof three coils, according to another embodiment of the invention. Thestructure is essentially symmetrical about axis A-A. Composite sectionsare provided, each comprising a coil 10 adhesively bonded on itsradially outer surface to a radially inner surface of an annular supportsection 12. In this arrangement, no radial flanges 14 need be providedon the annular sections 12. A spacer ring 30 is provided betweenadjacent annular sections 12. Each spacer ring 30 has aradially-inwardly directed flange 32, which is accurately dimensioned toensure correct alignment and spacing of coils 10 when positioned betweenadjacent annular sections 12.

FIG. 2B shows a cut-away view of a composite element, comprising a coil10 and an annular section 12 as used in the embodiment of FIG. 2A. Acrust of resin-impregnated filler material may be provided at a radialposition between the winding and the annular section.

At axial ends of the structure, retaining rings 34 are provided. Theseeach have a radially-inwardly directed flange 36, which may not need tobe accurately dimensioned, but should be flat, so as to align correctlywith the adjacent annular section. The retaining rings each have aradially-outwardly directed flange 38 which carries through-holes in amanner similar to that described with reference to FIGS. 1A-1C.

FIG. 2C shows a cut-away view of a spacer ring 30 as used in theembodiment of FIG. 2A. The retaining rings 30 each have at least oneradially-outwardly directed flange 40, which carries through-holes 18 ina manner similar to that described with reference to FIGS. 1A-1C. Asillustrated, in a preferred embodiment, each spacer ring 30 has tworadially-outwardly directed flanges 40, one at each axial extremity ofthe spacer ring. The holes in the two flanges may or may not be aligned,as preferred for the mechanical assembly arrangement chosen.

Bolts, or tie-rods with end fasteners, or similar arrangements, are usedto urge the retaining rings 34 towards one another, to compress thestructure and to hold the coils 10 in their respective relativepositions. The tie-rods 20 or bolts may extend the whole length of themagnet assembly, and be fastened only at their ends, as shown at 24 inFIG. 1B, so that the retaining rings 34 are directly linked togetherwith tie-rods or bolts, with the tie-rods or bolts simply passingthrough the through-holes in the flanges 40 of the spacer rings.

Alternatively, some or all of the spacer rings 30 are provided with oneor two radially-outwardly extending flanges 40, which each carry anumber of through-holes 18. Bolts or tie-rods with end fasteners attachthe retaining rings to the correspondingly adjacent spacer rings; andattach adjacent spacer rings together, as schematically illustrated at41 in FIG. 2A.

Separation and alignment of coils 10 is determined by the correctpositioning of each coil within the corresponding annular section 12,the correct dimensioning of the annular section 12 and the thickness ofthe radially-inwardly directed flanges 32.

The magnet coils are required to have a specific alignment with respectto each other to create the series magnetic field harmonics to createthe overall shape of the homogeneous (imaging) region of the magnetfield. Both the axial and concentric alignment of the coils must beconsidered during manufacture. If each coil 10 is axially andconcentrically aligned to the corresponding annular section 12, towithin a required tolerance, and the angular sections and spacers aredimensioned as required then the composite sections can be assembledwith the coils positioned correctly. Any variance of these dimensionswould require an alignment jig, which may be used either as a check ofthe coil alignment after assembly or to actively guide the correctalignment of the composite sections during the assembly.

Assembly of the structure of FIG. 2A may proceed as follows. Analignment jig is used to ensure correct axial alignment of the coilsduring assembly. For example, the alignment jig may comprise a number ofdemountable mandrels mounted on an axis aligned with the axis A-A of themagnet assembly. A first coil 10, adhesively mounted within its annularsection 12, is placed on the alignment jig. A spacer ring 30 is thenplaced over the first coil. Assuming that the spacer ring isappropriately manufactured, it is not necessary for the alignment jig tocontrol the positioning of the spacer ring. The spacer ring will lie onthe annular section 12, but small tolerance in its radial alignment isunlikely to be detrimental to the performance of the magnet as a whole.A next coil 10 is then stacked on top of the spacer ring, and is alignedaccording to the alignment jig. This process continues until all coilsand spacer rings have been stacked. If relevant, bolts or tie-rods areused to join spacer rings together, passing through holes inradially-outwardly extending flanges of the spacer rings. Retainingrings 34 are placed at axially outer ends of the assembly. One retainingring 34 may be stacked prior to the first coil; alternatively, bothretaining rings 34 may be placed in position after the coils and spacerrings have been positioned. Bolts or tie-rods are then positionedthrough through-holes in the radially outwardly-extending flanges 40 toretain the coils 10 in position, as described above. According to theembodiment in question, the bolts or tie-rods may extend between theretaining rings 34; or just between a spacer ring 30 and an adjacentspacer ring 30 or retaining ring 34. Radially-inwardly directed flanges32 are accurately dimensioned, for example being ground to a closetolerance on each of their axially-directed faces. This ensures correctaxial positioning of the coils when the structure is assembled. Theannular sections 12 may all be of a same axial extent d, with differentaxial spacing of the coils provided, as desired, by appropriatethickness e of the radially-inwardly directed flanges 32 of the spacerrings 40.

Advantages provided by such an embodiment include the following. Directmanual or mechanical handling of the coils is not required—each coil ishandled by the corresponding annular section 12. The use of an alignmentjig when stacking the coils and assembling the structure of FIG. 2ensures that the coils are accurately aligned, and ensures consistentand repeatable alignment of coils within magnet assemblies. If one coilis damaged, during assembly or later on, it is a relatively simple taskto release the retainers 24; 41, remove a retaining ring 34, coils 10,and spacer rings 30 for access to the damaged coil, and to replace thedamaged coil. The remaining coils and spacer rings and retaining ringmay then be replaced, to re-construct the magnet assembly. The alignmentjig should be used for alignment of coils 10 during re-construction ofthe magnet coil assembly.

Coils with Lugs Attached to Radially Outer Surface

FIG. 3A shows an axial view of a coil 10 with attached lugs 42,according to a further embodiment of the present invention. In thisembodiment, no annular sections are bonded to the coils 10. Instead,separate lugs 42 having through-holes 44 are attached to the radiallyouter surface 43 of each coil, circumferentially spaced around theradially outer surface. A magnet assembly may then be assembled bypassing retaining rods 20 through through-holes 44 in the lugs, andthrough spacers 22 such as shown and described with respect to FIGS. 1Cand 1E.

Preferably, the outer surface of each winding 54 is protected by a crustlayer 56 of glass cloth or glass beads, impregnated with thethermosetting resin used for impregnating the coils. This protects thewire of the windings from mechanical and electrical interaction with thelugs 42.

In an example method of manufacture, the lugs 42 could be positionedwithin the mold for resin impregnation, and bonded to the coil bythermosetting resin in the coil impregnation step. This has been foundto provide a reliable, high-strength bond between the coil and the lugs.In FIG. 3B, a cut-away view is shown of a coil comprising winding 54,with a crust 56 of resin-impregnated glass beads covering the radiallyouter surface of the coil. As lugs 42 are housed within the mold usedfor impregnation, they can be partially embedded within the crust 56,which provides a secure adhesive bond between each lug and the coil.

Alternatively, the coil may be impregnated in the conventional mannerwith thermosetting resin in a mold; the resin allowed or caused to cure;the coil removed from the mold, and lugs 42 then bonded to the radiallyouter surface of the coil using an adhesive. FIGS. 3C and 3Drespectively show a perspective view of a suitable lug 42 with a cavity46 useful for accommodating the adhesive 48 and a cutaway view of a lug42 adhesively mounted to the radially outer surface of a coil 10, withan adhesive 48.

FIG. 3E schematically illustrates an example arrangement for adhesivelybonding lugs 42 to coils 10. In this embodiment, the lug 42 extendsaxially the length of the coil 10. A cavity 46 is provided on theradially inner surface of the lug, to accommodate an adhesive 48 whichis used to bond the lug 42 to the coil 10. A channel 50 is providedthrough the material of lug 42 into cavity 46, to enable theintroduction of the adhesive into the cavity. The lug is placed on theradially outer surface 43 of the coil 10; adhesive is introduced throughchannel 50 into cavity 46, for example using a syringe. The adhesive isthen allowed to cure, providing a secure adhesive bond between the lug42 and the coil 10. Also illustrated in FIG. 3E is an extension piece52, an optional part of the lug which extends onto an axial end of thecoil 10. This allows simple manual axial alignment of the lug onto thecoil by pressing the extension piece against the axial end face of thecoil. The extension piece(s) may also assist in restraining the coilagainst the forces it encounters in use. Such forces would otherwisehave to be borne only by the strength of the bond holding the lug to thecoil.

In alternative embodiments, the lugs may have essentially rectangularradial cross-sections, having an axial length equal to the axial lengthof the coil.

FIG. 3F shows a partial view of an assembly of coils 10 according tothis embodiment of the invention, taken in a plane through holes 44,such as shown at IIIE in FIGS. 3B and 3D. As shown, the coils 10 may beassembled together by passing bolts or tie-rods 20 with end fastenings24 through the through-holes 44 in the lugs 42 and clamping the coilsinto position using the lugs and spacers 22. In these embodiments, itmay not be necessary to use spacers 22 of large radial cross-section,such as that shown in FIG. 1E, as the spacers will bear against thelugs, which may themselves be built of mechanically strong material suchas aluminum, with the interface with the coils 10 being through thelarger interface surface of the lug on the coil. However, use of largeradial cross-section spacers may advantageously improve the rigidity ofthe assembly as a whole.

Assembly of the arrangement of FIG. 3F is performed in a manneranalogous to that described with reference to FIGS. 1A-1C. An alignmentjig is used to ensure axial alignment of the coils, while the length ofspacers 22 determines the relative axial positions of the coils. As withthe arrangement of FIGS. 1A-1C, the spacers 22 are carefullydimensioned, for example by milling their axial ends.

Suitable materials for the lugs include aluminum and compositematerials. It has been found that aluminum and various compositematerials have acceptable surface properties for bonding to epoxyadhesives and are therefore expected to have acceptable properties foruse in this application taking into consideration thermal matching andmechanical properties of the specific parts.

In an example, a coil impregnated with an epoxy resin may be bonded toan aluminum lug using an epoxy adhesive.

A crust of epoxy-impregnated cloth, or glass beads, or similar, may beprovided over the radially outer surface of coils. These impregnatedcrusts of the coils may be machined to provide both axial andcircumferential alignment for the lugs before bonding lugs onto theradially outer surface of the crust.

In alternative arrangements, lugs may be formed in-situ, e.g. by makingsuitably-shaped cavities in the impregnation mold, and filling them withglass fiber cloth or other filler material before performing theimpregnation step. The resulting lugs are light, non-magnetic and a partof the coil structure itself, having a very high effective bondstrength.

The arrangement of FIGS. 3A-3C has the same advantages as thearrangements of FIGS. 1A-1C and FIG. 2 in respect of each of dismantlingfor replacement of a defective coil.

Coils with Molded in Inserts in Crust

FIG. 4 shows a cut-away view of a coil according to another embodimentof the invention. In this embodiment, the mechanical retaining structurewhich is adhesively bonded to the radially outer surface of the coil 10is a thickened crust 56 formed over the radially outer surface of thewinding 54. Through-holes 60 are provided through the crust. These arepreferably formed by including tubular inserts 62 within the crustduring the impregnation step. Alternatively, the crust may be formedwithout through-holes; then, over-sized holes may be drilled through thecrust, and the inserts 62 adhesively bonded into the crust. In otherembodiments, solid metal inserts are provided, which are drilled withthrough-holes after the impregnation step is complete. In yet furthervariations, no inserts are provided, and through-holes are simply moldedor drilled into the crust.

In a preferred embodiment, the wire 54 of the coil is wound onto a moldformer, and placed in a mold for resin impregnation, the mold havingsufficient space on the radially outer surface of the wire toaccommodate the crust 56. A number of inserts are added into the mold atappropriate locations, and remaining space within the mold filled withglass beads, glass cloth or another suitable filler material. The moldis closed, and the wire and crust is then resin impregnated, resultingin the structure shown in FIG. 4. The through-holes 60 may be filledwith a removable filling material such as wax or modeling clay toprevent the impregnating resin from blocking the through-holes. Featuresmay be provided in the mold to ensure that the inserts 62 are placed,and remain, in their correct positions. In addition, or alternatively, aweb (not illustrated) may be provided to ensure that the inserts 62 arecorrectly positioned relative to one another and relative to the winding54.

Such coil structures may be assembled together into a complete magnetassembly using tie bars or bolts with spacers, as explained withreference to other embodiments. Preferably, spacers with large axialsurface area, such as shown in FIG. 1E, should be used as these willspread the load applied to the crust and provide improved rigidity tothe magnet structure as a whole.

Batch Process Tool

FIG. 5 shows a part-axial cross section through a tooling arrangementwhich may be used in a method according to the present invention toperform resin impregnation and bonding of multiple coils in a singleoperation. As shown, multiple windings 54 are would onto respective moldjournals 70. As illustrated, these may be axially divided multi-partjournals for each coil, allowing each journal to be removed from itscorresponding coil once the impregnation step is complete. An advantagein using axially divided multi-part journals 70 is that it is simple toprovide runners 72 between the parts of the journals, allowingimpregnating resin to access the coils through the journal.

The outer parts 74 of the mold may be the annular support elementsdescribed above, which are mechanically joined to coils 10 in thefinished structure. In the illustrated arrangement, the outer parts 74each extend axially beyond the winding 54 of each respective coil, andso may be used in an assembly such as illustrated in FIGS. 2A-2C.Alternatively, by appropriately selecting the axial extent of eachannular support element 74, a magnet assembly of the present inventionmay be assembled without requiring spacers between the annular supportelements. FIG. 6 shows such an embodiment, and is described below.

Seals 76, such as elastomer rings, are provided between the annularsupport elements 74 to prevent resin from leaking between the supportelements. The journals 70, windings 54 and annular support elements 74are surrounded by a resin trough 80, and resin introduced under vacuumthrough the runners 72 to impregnate the windings 54 and bond them tothe annular support elements 74. Alternatively, the annular supportelements 74, linked by seals, may function as the outer section of thetrough sealed to a removable inner trough section, thus reducing theamount of excess resin. Preferably, an interface layer such as woundglass fiber cloth is provided between the winding and the annularsupport element 74. Voids 78 facilitate the flow of resin. The surfacesof the annular support elements exposed to the voids are preferablycoated with a release material such as PTFE polytetrafluoroethylene tofacilitate removal of excess resin once the impregnation step iscomplete. The surface of the annular support structure which interfaceswith the winding may be prepared in a manner which improves bonding bythe resin: for example, sandblasting, gritblasting or anodizing, if ofaluminum.

Once the resin has been introduced, and caused or allowed to cure, thetooling is dismantled, and excess resin removed.

FIG. 6 illustrates an axial cross-section of a magnet according to anembodiment of the invention, in which the coils may be prepared asdescribed with reference to FIG. 5. A number of coils 10 on annularsupport sections 74 are aligned co-axially about magnet axis A-A,preferably being stacked using a jig as described above. Retaining rings34, as described with reference to FIG. 2A, are placed at each end ofthe stack, and tie-rods 20 pass through respective holes in theretaining rings 34, and are fastened in place by fasteners 24 whichapply tension to tie rods 20 to compress the annular support sections 74and retain them in position. In this embodiment, there is no need toprovide separate spacers of any sort between the annular supportsections, as the axial extent of each support section is determined toprovide the correct coil spacing once the magnet is assembled. It may bepreferred to provide interlocking features on faces of the annularsupport sections, such as a raised ridge around one axial end surface ofeach support section, and a corresponding trough in the other axial endsurface of each support section—illustrated for example at 74 a in FIG.6. If these ridges and troughs are accurately machined, it may bepossible to assemble the magnet without use of the jig, simply bystacking the coils on top of one another, ensuring that the ridges andtroughs in the annular section are correctly aligned. In an alternativearrangement, the tie-rods may pass through holes in the support sectionspositioned on the radially outer surface of the support sections, forexample in lugs formed on the radially outer surfaces of the supportsections.

The Batch Process Tool arrangement of FIGS. 5 and 6, and the associatedmethods described above, may be used for winding and impregnating acomplete magnet, with the coil positions defined by the windingjournals, and bonded onto the support structure. During the process, thesupport structure including journals 70 is then disassembled. Thejournals and any other parts of the support structure are removed fromthe supported coils, and the coils 10 bonded to their respectivesupports 74 are reassembled, typically in the same sequence as they werewhen impregnated, to form a magnet structure with the correct coilpositions.

Segmented Outer Support Sections

FIG. 7 illustrates an axial cross-section through an impregnation toolarrangement which resembles that of FIG. 5. In this arrangement, theannular support sections 80 are made up of at least two, and preferablyat least three, arcuate parts. FIG. 8 generally illustrates an annularsupport section 80 made up of two arcuate parts 82. Each arcuate part isprovided with features 84 enabling them to be joined together to formcomplete annular support sections 80. In the example shown in FIG. 7,those features 84 may be lugs, each having a through-hole enabling boltsto be passed through adjacent lugs of adjacent sections to form asupport section 80. Such an arrangement is advantageous in that, oncethe coils 10 have been assembled into a magnet structure, the lugs maybe used to attach the magnet to a support structure, or to attach outercoils to the magnet.

In the arrangement shown in FIG. 7, a number of axially divided journalpieces 86 are retained together, for example by clamping. A number ofwindings 54 are wound into journal cavities. Layers of filler material,such as glass fiber cloth 88 may be wound over the windings 54. Theannular support sections 80 are then assembled around the windings 54,over the filler material 88, if present, by joining arcuate parts 82together. The resulting assembly is placed within a resin trough. In theillustrated example, the “trough” in fact only has an inner wall 90 anda base 92, the outer wall being effectively provided by the annularsupport sections 80. Seals 94, such as elastomer rings, are preferablyprovided between the annular support sections 80, and between the troughbase 92 and the lower annular support section 80. Resin 96 is thenintroduced, under vacuum, into the trough, and permeates resin flowpaths 98 between the journal pieces, as shown in the enlarged extract inFIG. 7. The resin impregnates the windings 54, the filler material 88,if any, and causes adhesive bonding of the resulting coil to the outersupport section. The resin is caused or allowed to cure. Surfaces of thejournal pieces 86 and surfaces of the annular support section which arenot to be bonded to the coil are preferably coated with a releasematerial such as PTFE polytetrafluoroethylene to facilitate disassemblyof the tooling once the impregnation step is complete.

This apparatus and the associated method described above provideswinding and impregnating of a set of identical coils, which maysubsequently be matched with different coils, which have beenmanufactured in a similar manner, to form a final magnet assembly.

Simplified Winding and Impregnation Tooling

FIG. 9 shows a partial radial cross-section through simplified toolingwhich may be used in a method of the present invention for preparingcoils for assembly into a magnet structure of the present invention. Inthis example, a single journal is provided, made up of two axiallydivided parts 100, having a runner 102 of small cross-sectional areadefined between them.

An annular section 104 having runners 106 defined in it is placed over awinding 54 wound into the journal, and any filler material 88 which maybe present. A thermosetting resin is introduced through runners 106 toimpregnate the filler material 88 and the winding 54. Air and excessthermosetting resin may exit the journal through the runner 102.Alternatively, the resin impregnation may be carried out under vacuum. Atool 108 may be temporarily placed over runner 102 to collect excessresin. The tool may be sealed to the journal with seal 110. In theillustrated arrangement, hoses are attached to runners 106 to carry theresin to the coil. Alternatively, resin impregnation may be performedusing a resin trough, as is conventional. The impregnating resin iscaused, or allowed, to cure. The journal pieces are then removed fromthe coil structure to leave the coil 10 comprising impregnated winding54 adhesively bonded to the annular support section 104.

Such a simplified tooling arrangement may be used to form coilassemblies according to any of the described embodiments of the presentinvention. Embodiments such as shown in FIG. 4, where the annularmounting section is composed of impregnated filler material may beformed by a similar method, in which the annular part 104 of the mold iscoated with a release coating, to enable removal of the coil from theannular part.

Spindler and Hoyer Kit

FIG. 11 illustrates a coil 10 mounted upon a square support section 130.This assembly may be prepared by any of the methods previouslydescribed, For example by manufacturing a resin-impregnated coil by anyconventional method, aligning it with the square support section 130 andbonding the coil to the support section, for example by a second resinimpregnation step, possibly interposing a filler layer such as glasscloth or glass beads between the radially outer surface of the coil andthe adjacent surface of the support section 130 before the secondimpregnation step. Alternatively, the square support section 130 may beused as a part of the mold during the first impregnation step. In thatcase, the adjacent surface of the support section 130 is not coated witha release agent, but on the contrary may be roughened by sandblasting,gritblasting or anodizing as appropriate to ensure secure bonding of thecoil to the support section.

Through-holes 132 may be provided through the support section asillustrated. For a square support section as illustrated, it may beparticularly convenient to provide four through-holes 132, one near eachcorner of the square support section; or a multiple of fourthrough-holes, evenly distributed about the four corners.

FIG. 12 illustrates an arrangement, similar to that shown in FIG. 3F,which may be used to assemble supported coils such as that illustratedin FIG. 11 into a magnet assembly. As illustrated, and as in the exampleof FIG. 3F, hollow spacers 22 may be provided, over tie-rods 20, toensure correct spacing of the coils 10 on the support sections 130.Alternatively, tie rods 20 may be threaded along their length, forexample being aluminum or stainless steel studding, and nuts and washers24 may be provided on both sides of each support section 130 to holdeach coil in place. Such an arrangement has the advantage that thepositions of the coils 10 may easily be adjusted during assembly.

FIG. 13 shows another adjustable coil retention arrangement which may beused in assembling supported coils such as that illustrated in FIG. 11into a magnet assembly. Tie rods 20 may be threaded or unthreaded inthis arrangement. At each through-hole 132, a threaded hole 134 isprovided, to intercept through-hole 132 approximately at right angles.Screws 136 are provided in the threaded holes. Once a coil is aligned inposition, for example using a jig as described previously, the screws136 are tightened onto the tie-rods to hold the coil in position.Spacers 22 would not then be required. The ends of the screws 134 may betapered to provide more secure retention of the coils in position on thetie rods.

Similar arrangements have been used, with twelve aluminum M12 bolts eachhaving a tensile load of about 1 Tonne have been found sufficient torestrain an axial force on the end coils of a 0.5 Tesla magnet of theorder of 10 Tonnes. For high field magnet designs, the number and sizeof tie bars may be increased to provide the required structuralstrength.

In an improvement of this arrangement, adjustment of the coilconcentricity may be enabled by using three or more bolts 138 at eachpinch point, for example as illustrated in FIG. 14, where four bolts 138are provided, perpendicular to one another and to the tie bar, andprovided through threaded holes in a support ring 140 provided for thepurpose on at least one side of the support section 130. Suchembodiments make use of the tie bars without the need for spacers 22. Inan alternative embodiment the support sections 130 may be extended outradially to support an active shielding coil (not illustrated).

Adjuster for Coil Alignment

The above-described assembly methods describe the use of a jig to ensurecorrect alignment and assembly of the coils. It may be found necessaryto provide a mechanical adjustment between coils, while the assembly ison the jig or after it has been removed from the jig. For example, anelectric current can be passed through the metal cross-section in thewire and the resulting magnetic field measured. The axial coil positionsof the magnet can then be optimized for the desired magnetic fieldharmonics. FIG. 10 illustrates an example of a mechanical coil alignmentadjuster as may be used in some embodiments of the present invention. Itmay be regarded as an adjustable spacer. Between two annular supportsections 120, a retainer 122 carries a conical rotary adjuster 124. Aradially inner end of the adjuster has a conical surface 128, whichbears upon correspondingly tapered surfaces of the adjacent annularsupport sections 120. The adjuster has a threaded shaft which passesthrough a threaded hole in the retainer 122. The adjuster preferably hasa head configured for simple operation with a hand tool: for example, ahexagonal head for turning with a spanner; a recessed hexagon forturning with an Allen key, a recess to receive a screwdriver and so on.At least three adjusters should be provided around the circumference ofthe support sections, at any given axial position.

The coil alignment adjuster may be operated as follows. With the tierods 20 under little or no tension, the head of each adjuster may beturned to adjust the relative alignment of coils. Moving the adjusterradially inward will push the support sections and the attached coilsfurther apart at that circumferential position, while moving theadjuster radially outward will allow the support sections and theattached coils to move closer together at that circumferential position.

In an example embodiment, the conical surface of the adjuster has anincluded angle of 4°, allowing precise adjustment of coil alignment. Theretainer 122 may be a complete ring, placed in position as the coils areassembled together. Alternatively, individual retainer bosses may bepositioned at required positions, preferably equally spaced, around thecircumference of the annular retainer sections at a particular axialposition. The bosses may be attached to the annular retainer sections120 by a suitable arrangement such as bolts in threaded holes in theannular retainer sections.

In a development of this idea, the conical rotary adjuster 124 may bereplaced by a tapered bar, which is radially pressed into the taperedgap between annular support sections, or radially released from thetapered gap as desired to achieve correct coil positioning. The use of atapered bar will have the advantage of providing a greater contactsurface area with the annular support section, reducing mechanical pointloading on the annular support sections 120. The tapered bar may bepositioned by an adjustable strap running circumferentially around theretaining sections and the bar, or by threaded adjusters similar to thatillustrated in FIG. 10, which bear on a radially outer surface of thebar.

While the present invention has been described with reference to alimited number of specific embodiments, it will be apparent to thoseskilled in the art that numerous variations and amendments are possible,within the scope of the invention as defined by the appended claims.

For example, while the present invention has been specifically describedwith reference to cylindrical superconducting magnets for MRI systems,it may be applied to cylindrical electromagnets for any purpose, whetherthey are superconducting or resistive.

A full scale prototype magnet has been manufactured and tested with endcoils that have been bonded to the support structure in a two stageimpregnation process and proved operational at 3 Tesla.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

We claim as our invention:
 1. A superconducting solenoidal magnetassembly comprising: a plurality of superconducting solenoidal coilsconcentrically aligned along a longitudinal axis proceeding through theplurality of coils, each superconducting, solenoidal coil comprising awinding impregnated with a resin, the resin of each superconductingsolenoidal coil forming two annular outer surfaces that areperpendicular to said longitudinal axis and a radially outer surface,between said two annular surfaces that proceeds around said longitudinalaxis; a plurality of annular support sections equal in number to saidplurality of superconducting solenoidal coils, said plurality of annularsupport sections also being concentrically aligned with, and proceedingalong, said longitudinal axis, respectively in one-to-one correspondencewith said plurality of superconducting solenoidal coils, each of saidannular support sections comprising a radially outer surface withrespect to said longitudinal axis; adhesive forming an adhesive bondbetween the radially outer surface of the resin of each superconductingsolenoid coil and the corresponding one of said annular supportsections; a mechanical restraint acting axially parallel to saidlongitudinal axis on the respective annular support sections to hold therespective annular support sections in fixed axial relative positionsalong said longitudinal axis to form a stack of said superconductingsolenoidal coils, said stack having opposite axial ends with respect tosaid longitudinal axis; and said mechanical constraint comprising anannular retaining ring at each of said opposite axial ends of saidstack, each annular retaining ring comprising a first ring portion thatabuts the annular support section at the respective axial end of thestack, and a second ring portion that projects radially outwardly beyondthe radially outer surfaces of the annular support sections, each secondannular portion of each annular retaining ring having holes therein,with the holes in the respective annular retaining rings being alignedwith each other, and said mechanical constraint comprising tie rodsproceeding through the aligned holes in the respective annular retainingrings and proceeding parallel to said longitudinal axis, each tie rodcomprising a threaded portion that extends longitudinally beyond therespective second annular portions of the respective annular retainingrings, with bolts being respectively tightened on said threaded portionsto hold the respective annular support sections in said fixed axialrelative positions along said longitudinal axis to form said stack.
 2. Asuperconducting solenoidal magnet assembly according to claim 1 whereinthe annular support sections each have an axial extent that is longerthan the axial extent of the associated superconducting solenoidal coil,the annular support sections being axially restrained against oneanother, such that a spacing between the respective superconductingsolenoidal coils is defined by the axial length of the annular supportsections.
 3. A superconducting solenoidal magnet assembly as claimed inclaim 1 wherein each of said annular support sections has first andsecond oppositely disposed edges, and wherein said mechanical restraintis configured to act on the annular support sections to cause therespective annular support sections to successively abut one another,thereby forming a plurality of pairs of abutting annular sections with afirst edge of one of said abutting annular sections in each pairabutting the second edge of the other annular section in each pair, andwherein said first edge of each of said abutting pairs of annularsections has a first non-planar profile and the second edge of each ofsaid abutting pairs of annular sections has a second non-planar profilethat mates with said first profile to interlock the respective pair ofabutting annular sections with each other.